Gigantoraptor erlianensis stood more than 16 feet (five meters) tall and weighed a ton and a half, or roughly 1,400 kilograms. Featuring a toothless beak on its head and a short tail for balance, the enormous birdlike dinosaur measured more than 26 feet (eight meters) in length. Living more than 65 million years ago at the end of the Cretaceous period, this big bird precursor has complicated the seemingly shrinking descent from Archaeopteryx to the modern sparrow.

Paleontologist Xing Xu of Beijing's Institute of Vertebrate Paleontology and Paleoanthropology and colleagues discovered the animal while prospecting for fossils in the Erlian Basin of the Gobi Desert in north-central China. Based on its size, the paleontologists initially classified it as member of the tyrannosaur lineage, but bits of beak, leg and other bones revealed that it more properly belonged to the oviraptorosauria group, heretofore a grouping of small, feathered creatures weighing only a few pounds. "It is the largest known beaked dinosaur," Xu says. Adds paleontologist Mark Norell of the American Museum of Natural History in New York City: "I was just flabbergasted when I saw it because it was so big."

The finding complicates the evolutionary descent of birds from dinosaurs. "Progressively from within advanced theropods you get smaller and smaller towards birds," Norell says. But "after some species originate and spring off the bird line, you get secondary gigantism."

No one can say why for sure. "Big size has some advantages such as having fewer predators and having more food resources that are unavailable for small animals," Xu notes. But he adds that early oviraptorosaurs, this animal's ancestors, "are among the smallest dinosaurs."

"Maybe it's something about the environment," Norell speculates. "This is a very nascent stage of starting to understand the evolutionary dynamics of the animal."

Already, Gigantoraptor is proving very odd, according to the paper detailing the find published in Nature. It bears a ridged humerus (upper arm bone) never before observed in a dinosaur as well as an enormous but birdlike femur (thighbone). "Maybe it has some sort of birdlike running mechanism," Xu says. Norell adds: "We don't know the biomechanical advantage this would confer. But I don't think it's hopping around at that size."

It also is not clear whether the creature was feathered, though Xu speculates it was like most other animals in its lineage. If so, it would be by far the largest known feathered animal of all time, tripling the mass of Stirton's thunderbird, which lived in Australia roughly eight million years ago. "Certainly the babies would have had to have feathers until they reached thermal regulation with the environment," Norell notes.

So what did this prehistoric "Big Bird" eat? Another mystery, as the animal bore no teeth in its jaw, had a small head and long neck (common in herbivores) but also sharp claws (common in carnivores). "[It] could be omnivorous," Xu says. Norell adds: "Lots of things that have no teeth like hawks and eagles are pretty efficient carnivores." It seems the world of dinosaurs—and bird ancestors—was as diverse as that of their latter-day descendants.